When you want to reply to someone who has left a reply in the thread (ie she occupies a "floor"), you can click the reply or the comment at the bottom of her floor and leave your message to her. She can get a notice asking her to come back to see what you have written.
When you want to talk to someone who does not occupy a floor in the thread (eg she has just left a comment, or a rating, or you just want to ask someone to come here), you can use the @ function. To use the @ function, it must be a new post (ie you cannot edit an old post), and you must click the function icon to fill in the username instead of just typing it out. When you do it correctly, you can see a hyperlink when mouseover after posting it.
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